A new documentary explains how two Jewish families escaped the Nazis by hiding in caves for 17 months.

It is unimaginable to think that 38 Ukrainian Jews lived for 500-plus days in underground caves to escape Nazi persecution during World War II. But as the remarkable documentary “No Place on Earth” shows, the Stermer and Wexler families did exactly that.

This extraordinary story begins when Chris Nicola, an amateur cave explorer from New York, discovers medicine bottles, buttons, and shoes in a Ukrainian gypsum grotto he visited when he hoped to uncover his own family history. These artifacts were evidence that people lived underground in the Verteba Caves. And so he investigated further, discovering the story of matriarch Esther Stermer’s family.

“No Place on Earth” interviews four elderly survivors — Saul and Sam Stermer, and Sonia and Sima Dodyk — who were children in the years when the Germany and Russia divided Poland and the Nazis were capturing and killing Jews. To stay alive, the two families entered the caves, building beds, collecting water from “drippings” — one glass per family per day — and digging passages through the earth. Reenactments dramatize much of the Stermer and Dodyks’ experiences since original footage is not available.

But it is the memories these survivors have about their lives underground that provide the most fascinating parts of this documentary. From stories of Germans entering the cave and shooting at the people inside, to horrific recollections of death and survivor’s guilt, this triumph of the human spirit story is deeply affecting. Likewise, anecdotes about the men going out and foraging for food, while the women were not allowed to leave, answer questions about how they were able to maintain a life underground for as long as they did. The women’s fears that the men might not return are palpable — especially when it is revealed that chopping wood to build fires was dangerous because it created noise that might cause the hiding families to be discovered.

If “No Place on Earth” glosses over the fights and arguments that took place during the 17 months underground, the film does show the Stermer and Dodyks returning to the Ukraine as seniors to “thank” the caves for saving their lives. And watching one elderly survivor descend again into the caves is almost as amazing as seeing the children react to sunlight after being in darkness for so long.